Advanced, high energy density batteries are required for use in space, military, communication and automotive applications. Certain jurisdictions, such as California, have mandated that an increasing percentage of automobiles must be powered by electricity within the next few years. The lead-acid battery, though reliable and capable of many recharge cycles, is too heavy and has too low an energy to weight ratio. State of the art Ag--Zn and Ni--Cd batteries have poor charge retention properties and are also too heavy and bulky for use on space missions and in some cases do not meet the life and environmental requirements for the missions.
Ambient temperature, secondary lithium batteries have several intrinsic and potential advantages including higher energy density, longer active shelf life, and lower self discharge over conventional Ni--Cd, Pb--acid and Ag--Zn batteries. Successful development of these batteries will yield large pay-offs such as a 2-3 2 fold increase in energy storage capability and a longer active shelf life of 2 to 4 years over Ni--Cd. These cells are most suitable for small spacecraft application requiring less than 1 kW power. Secondary lithium batteries are presently being considered for a number of advanced planetary applications such as: planetary rovers (Mars Rover, Lunar Rover), planetary spacecraft/probes (MESUR, AIM, ACME Mercury Orbiter) and penetrators. These batteries may also be attractive for astronaut equipment, and Geo-SYN spacecraft.
Secondary lithium cells under development employ lithium metal or lithium ions in carbon as the anode, a chalcogenide salt such as TiS.sub.2, MoS.sub.2, MoS.sub.3, NbSe.sub.3, V.sub.2 O.sub.5, Li.sub.x Mn.sub.2 O.sub.4, Li.sub.x CoO.sub.2, LiV.sub.3 O.sub.8 and Li.sub.x NiO.sub.2 as cathodes and liquid or solid electrolytes. During discharge of the cell, lithium metal is oxidized into lithium ions at the anode and lithium ions undergo an intercalation reaction at the cathode. During charge, reverse processes occur at each electrode.
Solid polymer electrolyte/lithium batteries using polyethylene oxide (PEO) and other organic polymers complexed with lithium salts as the electrolyte are under development. In addition, gel (polymer) electrolytes have received attention because of improved conductivity over the solid polymer electrolyte materials. These electrolytes have low transference number (0.3-0.5) for lithium cations leading to high concentration polarization and high interface resistance. Salt anions (BF.sub.4.sup.-, AsF.sub.6.sup.-, ClO.sub.4.sup.-, CF.sub.3 SO.sub.3.sup.-) contained in the polymer are not compatible with lithium and cause the lithium to degrade. The solid polymer electrolytes (PE) have low mechanical strength especially above 100.degree. C. The disadvantages of prior art PE's deter development of high power, high energy polymer lithium batteries for the following reasons. In the PE's the Li cation, which is complexed (bound) to the polymer has low mobility, while the uncomplexed anion moves faster. The activity of the salt anions with lithium results in a thick lithium passivating layer which has high resistance. Also, above 100.degree. C. the prior art PE's become soft and start to flow.
These problems were addressed by changing the mechanism for conduction of lithium ions, eliminating the non-compatible ions and using compatible ions such as halide and adding an inorganic filler as a reinforcing agent.
Addition of Al.sub.2 O.sub.3 (2) (3) has improved the mechanical strength of polymer electrolytes. However, the lithium transference number was low because the salt was not compatible with lithium ions, the salt concentration was too low and the Al.sub.2 O.sub.3 particles were too large. Solid lithium iodide (LiI) has good ionic conductivity and low electronic conductivity. Its lithium transference number is close to unity. Conduction is accomplished through a lithium vacancy mechanism (1). It was found that by mixing LiI and Al.sub.2 O.sub.3 powders and pressing them into a pellet an order of magnitude is gained in conductivity over pure LiI. This is due to the presence of Al.sup.3+ cations at the LiI interface which results in an increase in the Li.sup.+ vacancy concentration. The Li.sup.+ conduction is carried out in the LiI mainly at the LiI/Al.sub.2 O.sub.3 interface. However, a LiI-Al.sub.2 O.sub.3 pressed pellet is very brittle and has poor mechanical and shock properties. In practice thick pellets are required to avoid these problems. This principle was the basis for the solid state batteries used in low rate medical applications.